Simple filters

• Simple filters are created with HTTP::Proxy::BodyFilter::simple or HTTP::Proxy::HeaderFilter::simple.

• These are filter factories that return a filter object given the filtering routine

      $filter = HTTP::Proxy::BodyFilter::simple->new(
          sub { ${ $_[1] } =~ s/foo/bar/g; }
      );

• The routine must have the same signature as the filter() method.

• One can also use:

      $filter = HTTP::Proxy::BodyFilter::simple->new(
          filter => \&filter_sub,
          start  => \&start_sub,
      );

  to define the other standard methods.

• HTTP::Proxy::BodyFilter::simple and HTTP::Proxy::HeaderFilter::simple are conceptually identical